1. Field of the Invention
The present invention relates to a reproducing apparatus and a reproducing method in which when a run length limited code to be recorded on a disc is subjected to modulation in its width direction to superimpose a sub-data train, an offset upon detecting a signal level is reduced.
2. Description of the Related Art
A so-called pit top disperse recording method is incorporated by reference U.S. patent application Ser. No. 09/213012) filed in Dec. 16, 1998 by the same applicant of the present invention.
The above-mentioned pit top dispersion recording method is such a recording method in which a pit or land alternatively a mark or space is subjected to a modulation in its width direction to thereby superimpose and record a sub data train. The sub data train can be reproduced by detecting the sub data train from a fine modulation signal included in a reproduced signal and demodulating the same.
In a case where encryption is applied to a main data so as to protect a copy right, for example, the above sub data is a key for decoding the encryption or is a code to discriminate a normal disk from a pirated disk.
A first method to increase the detection sensitivity for the sub data train is such that, for recording the sub data, the number of pits or marks alternately lands or spaces, to which the pit top dispersion recording method is applied, is increased to thereby improve an S/N ratio (signal to noise ratio). Specifically, by increasing the number of pits or marks, or lands or spaces which are modulated in width direction on an optical disk, the S/N ratio is improved.
A second method to increase the detection sensitivity for the sub data train is such that the amount of a fine modulation signal to pits or marks alternately lands or spaces, to which the pit top dispersion recording method is applied, is increased to thereby improve an S/N ratio.
Specifically, by increasing the depth of concave portions or projection of convex portions pits or marks, or lands or spaces which are modulated in width direction on the optical disk, the S/N ratio is improved.
A third method to increase the detection sensitivity for the sub data train is such that, in order to detect pits or marks alternately lands or spaces to which the pit top dispersion recording method is applied, all the pits or marks alternately lands or spaces are sampled at a predetermined timing to correct a bias between generation probability of 1 and generation probability of 0 (difference between generation frequencies), thereby improving the S/N ratio.
Specifically, when a disk identification code SC1 described in the above-mentioned U.S. patent application Ser. No. 09/213012 is modulated on the basis of an M sequence random number data, by correcting the bias between generation probabilities of 0 and 1 within a predetermined period, it is possible to improve the S/N ratio.
A fourth method to increase the detection sensitivity for the sub data train is such that by normalizing an accumulation result of the generation number of 1 and the generation number of 0 within the predetermined period generated in the M sequence random number generation portion on the basis of the frequency of the above sampled value, it is possible to improve the S/N ratio.
However, the first to fourth methods for improving the S/N ration which is the detection sensitivity of the sub data train have the drawbacks as mentioned hereinbelow.
The first method has a disadvantage such that since the number of pits or marks, or lands or spaces, to which the pit top dispersion recording method is applied, is increased, it takes long period of time to detect the sub data train.
The second method has a disadvantage such that since the modulation amount is increased, the recorded sub data train causes a cross-talk on the main data train and easily exerts bad influence. Further, there is also a disadvantage such that when the modulation amount of the sub data train is increased, it can be easily demodulated. Thus, it is easily duplicated and altered a malicious third party, so that the effect for preventing the forgery and so on are lost.
The third method has a disadvantage such that since a timing of sampling depends on the pit or mark or land or space which are the main data train is determined irrespective of the sub data train, it is substantially impossible to correct the bias in the number of sampling results.
The fourth method has a disadvantage such that since there is required an arithmetic unit for carrying out a dividing for one accumulated result based on the accumulation result of the generation number of 0 and the generation number of 1 for performing the normalization, the circuit becomes large-scaled and complicated.